As for the coating materials for electric insulation, especially the embedded type compounds, those obtained by compounding an organopolysiloxane of the type which can be cured by condensation polymerization with a catalyst for the condensation polymerization, an inorganic powder, an organic solvent and so on have so far been used exclusively. However, it is usually required for curing such compounds to heat them up to temperatures higher than 150.degree. C. Even when powerful catalysts are used, it is as much as they can do to lower the curing temperature to about 120.degree. C. Consequently, it takes 16 hours or more to effect the curing, that is to say, the producing efficiency is low. Under these circumstances, there is little hope of increasing the productivity, and what is worse, satisfactory properties cannot always be obtained.
In spite of such conditions, there has been a growing request for compounds capable of being cured at 100.degree. C. or lower, preferably around 70.degree. C., with the recent diversification of electric parts to be embedded in one case, particularly with the increase in demand for the embedment of thermal fuse. This is because electric parts, especially thermal fuse, must not be damaged by the thermal curing.
From such a viewpoint, a method of causing an organopolysiloxane containing alkenyl groups and an organopolyhydrogensiloxane, which contains hydrogen atoms bonded directly to silicon atoms, to undergo an addition reaction in the presence of a platinum catalyst and then curing the resulting adduct has been carried out up to now.
In this case, it is necessary to get rid of all silanol groups by the condensation reaction in the process of producing the alkenyl group-containing organopolysiloxane to be used as a reactant. This is because there is a necessity for preventing such a disadvantage that aside from the foregoing addition reaction, the remaining silanol groups react with the organopolyhydrogensiloxane containing hydrogen atoms bonded directly to silicon atoms owing to the catalytic action of the platinum compound to cause dehydrogenation and condensation reaction, resulting in shortening of pot life. In conventional compositions, however, a strong alkali or acid had to be used as a catalyst in order to get rid of the above-described silanol groups, whereby polymerization of the alkenyl groups took place. Consequently, it was difficult to obtain the intended alkenyl group-containing organopolysiloxane.